Probing local thermal, mechanical, and optical properties utilizing dynamic cantilever response in contact mode atomic force microscopy

Understanding the behavior of materials and devices at the nanometer-scale is important because modern materials and devices have nanometer-scale features. Atomic force microscopy (AFM) is a powerful tool for studying nanometer-scale behavior due to excellent spatial resolution (tip radius < 25 nm) and the ability to measure dynamic surface deformation with sub-picometer precision. Measurement of dynamic surface deformation in response to a stimulus (e.g. heating or mechanical force) provides information about local material properties. This thesis presents three studies which use dynamic surface deformation measurements to investigate nanometer-scale thermomechanical, inverse-piezoelectric, infrared, and mechanical properties. The first study uses AFM to measure thermomechanical and inverse-piezoelectric deformation of biased AlGaN/GaN transistors. Deformation measurements during device heating reveal shifts in the thermomechanical strain fields within devices as bias conditions change. Deformation measurements without heating reveal bias dependence of inverse-piezoelectric deformation. Measurements validate an electro-thermo-mechanical finite element model, which predicts device stress and failure. The second study uses AFM to measure infrared absorption by observing thermomechanical deformation due to infrared light absorption. This thesis describes a novel implementation which enables two orders of magnitude improvement in sensitivity. Measurements of carbon nanotube absorption (diameters near 2 nm) and monolayer graphene demonstrate the effectiveness of this technique. The third study presents the design, fabrication, and implementation of micromechanical contact stiffness devices which provide a range of known contact stiffness. These devices are useful for calibrating dynamic cantilever response as a function of contact stiffness, which is critical for AFM measurements of mechanical properties. This study concludes with the calibration of an AFM cantilever for contact resonance AFM and subsequent measurement of contact stiffness and elastic modulus on three different polymers. The AFM elastic modulus measurements on polymer samples agree with comparable bulk measurements

Addressing thermal and environmental reliability in GaN based high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) have appeared as attractive candidates for high power, high frequency, and high temperature operation at microwave frequencies. In particular, these devices are being considered for use in the area of high RF power for microwave and millimeter wave communications transmitter applications at frequencies greater than 100 GHz and at temperatures greater than about 150 °C. However, there are concerns regarding the reliability of AlGaN/GaN HEMTs. First of all, thermal reliability is the chief concern since high channel temperatures significantly affect the lifetime of the devices. Therefore, it is necessary to find the solutions to decrease the temperature of AlGaN/GaN HEMTs. In this study, we explored the methods to reduce the channel temperature via high thermal conductivity diamond as substrates of GaN. Experimental verification of AlGaN/GaN HEMTs on diamond substrates was performed using micro-Raman spectroscopy, and investigation of the design space for devices was conducted using finite element analysis as well. In addition to the thermal impact on reliability, environmental effects can also play a role in device degradation. Using high density and pinhole free films deposited using atomic layer deposition, we also explore the use of ultra-thin barrier films for the protection of AlGaN/GaN HEMTs in high humidity and high temperature environments. The results show that it is possible to protect the devices from the effects of moisture under high negative gate bias stress testing, whereas devices, which were unprotected, failed under the same bias stress conditions. Thus, the use of the atomic layer deposition (ALD) coatings may provide added benefits in the protection and packaging of AlGaN/GaN HEMTs.M.S

Thermally Aware Design Approaches for High Power Density Ultra-Wide Bandgap Power Electronics

Ultra-wide bandgap (UWBG) semiconductors like β-type gallium oxide (β-Ga2O3) show promise for the development of next-generation high power density electronics devices such as RF and power electronics. The large bandgap (4.8 eV), high breakdown fields (8 MV/cm), and excellent thermal stability of β-Ga2O3 give promise to the production of low-loss power switching devices with large breakdown voltage, and potentially allows for high-temperature and deep space operation. However, a major drawback of β-Ga2O3 arises from its poor thermal conductivity, which results in devices with unacceptably high junction-to-package thermal resistance. While there is considerable promise for future devices made from UWBG materials, their adoption as a technology will hinge upon novel approaches to address heat dissipation at the die level which will enable high power density operation. The aims of this thesis are i) to develop novel thermal management strategies to reduce the junction-to-package thermal resistance for devices made from low thermal conductivity UWBG materials for both lateral and vertical devices, ii) to conduct an analysis of architectures for homoepitaxial β-Ga2O3 metal-oxide semiconductor field effect transistors (MOSFETs) to optimize the device thermal performance and verify experimentally, and iii) to optimize thermal management design for both steady-state and transient-state of UWBG transistors. Overall, the optimal thermally-aware design for vertical and lateral structures for steady-state and transient applications will be provided by investigating the device layout such as substrate orientation, configuration of electrodes (number of fingers, channel width, location of metallization pads), dielectric heat spreader, and thermal boundary conductance between metal and β-Ga2O3.Ph.D